Abbreviation of "Fuck you sayin", which is a shortened version of the rhetorical question "What the fuck is you saying?" Similar to FOH and FYM, this is a way to tell a person that you dont give two fucks about what they're saying to you rite now.

Me: The PS3 has Metal Gear. Playstation >>>>> Xbox

You: Yeah thats true. Except I have both systems, and Xbox has Halo, so FYS!!